A voltage controlled transistor such as MOSFET is used as a switching circuit for a power circuit. A digital circuit is sometimes used for controlling the voltage controlled transistor; generally the standard voltage of the digital circuit is about 5 V, but with advance in voltage reduction, some control circuits have the standard voltage of about 3 V. Therefore, an element having a threshold voltage of 5 V or more in a voltage controlled transistor has to be provided with a power source separately from a power source of the control circuit; even if the threshold voltage is 5 V or less, when the threshold voltage is close to the control circuit voltage, a charging speed for an input capacity of a gate goes down, hence to disadvantageously delay the switching speed. Further, a wide gap semiconductor described later includes a normally-on element and in order to switch off the element, a negative power source must be prepared.
In a circuit using switching, power loss varies depending on a transition speed of turning on/off of a voltage controlled transistor. The higher transition speed of a voltage controlled transistor is more preferable; for that purpose, it is necessary to control a gate voltage of the voltage controlled transistor speedily. In this respect, various kinds of resonant gate drive circuits have been proposed.
The conventional means so far known, however, cannot drive the voltage controlled transistor having a higher threshold voltage than that of a drive circuit with a single power source nor a normally-off element without using a negative power source, and further the conventional means is defective in a simple structure because it needs many switches for a drive circuit in order to restrain such a gate voltage caused by resonance that breaks down the voltage controlled transistor, with a complicated sequence of the drive circuit and with the increasing number of components forming the circuit.
A wide gap semiconductor of GaN, SiC, diamond, and the like is characterized by a superiority over that of Si in the important points of a semiconductor device such as dielectric breakdown voltage, electron mobility, and thermal conductivity. An AlGaN/GaN hetero HEMT, that is one of the GaN semiconductor devices, has attracted much attention and expectation, as it has high frequency property and low on-resistance property because of having a high electron mobility and carrier density. The HEMT or SiC transistor having the AlGaN/GaN hetero-structure has a property (normally-on) of flowing current when a gate voltage is not applied; therefore, in order to turn off the element or to stop the current, a negative voltage must be applied to the gate. Therefore, a negative power source is required; as the result, the number of the components for a circuit is increased and the wiring on a circuit substrate gets complicated.
As a circuit which saves the negative power source, such a method can be considered that a control signal is supplied to a gate of HEMT through a capacitor from a transmitter, and that a diode is arranged between the capacitor and the source of HEMT, to save the negative power source. This method, however, needs a diode and a capacitor having a large capacity 10 to 500 times more than the input capacity of the voltage controlled transistor externally, in addition to a semiconductor device; therefore the capacitor occupies a large volume on a substrate.
Alternatively, the drive circuit may be formed on the same chip as a semiconductor device of a single element. This, however, also needs a capacitor having a large capacity, which makes it difficult to design the above. Further, voltage oscillation (ringing) may occur due to the resultant parasitic inductance from the wiring from a drive circuit of a switching element to a gate of the switching element getting longer.